Consumer products such as communication devices and in particular wireless telephones have long become standard commodity.
In such systems, the transmitting side encodes the communicated data, while the receiving side decodes it. It will be appreciated that the transmitting and receiving sides may alternate according to the speaking side. Many of these devices use Adaptive Differential Pulse Code Modulation (ADPCM) codecs, which are waveform codecs, in which the encoder instead of quantizing the speech signal directly, quantizes the difference between the speech signal and a prediction that has been made of the speech signal. If the prediction is accurate then the difference between the real and predicted speech samples is of lower variance than the real speech samples, and is accurately quantized with fewer bits than would be needed to quantize the original speech samples. At the decoder side, the quantized difference signal is added to the predicted signal to give the reconstructed speech signal.
In order to improve and make the usage of the transmitting side and receiving side of a communication system more efficient, discontinuous transmission (DTX) may be used. DTX is a method for reducing transmission and thus optimizing the overall efficiency of wireless voice communications systems, by momentarily powering-down or muting any of the sides, when no voice activity is detected.
In a typical two-way conversation, each individual speaks on average slightly less than half of the time. If the transmitter signal is switched on only during periods of voice input, the duty cycle of the system can be cut to less than 50 percent on average. This conserves battery power and radiation, eases the workload of the components in the transmitter amplifiers, and reduces interference.
As explained above, in ADPCM the encoding and decoding are not stateless, but rather recently communicated data is used during encoding and decoding current data. Therefore, after the two sides do not communicate for a while, getting back into communication introduces a synchronization problem.
Some prior art solutions include halting all activity of the encoder and decoder so that they remain at the same state. The drawback of this solution is that it is hard to ensure that once communication resumes, encoding and decoding start at the same sample, since even a very small deviation causes severe noises.
Other solutions include encoding and decoding predetermined or artificial data such as null data. This solution is more stable, but still requires the encoder and decoder to fully operate even when there is no data is transmission.
Yet another solution relates to introducing comfort noise generated by a comfort noise generator (CNG) to the encoder and the decoder, and have the encoder and decoder operate on the comfort noise.
However, none of these solutions provides satisfactory synchronization as well as processing power savings by avoiding unnecessary encoding and decoding.
There is thus a need in the art for a method and system for synchronizing the two sides in ADPCM with TDX communication systems.